Master cylinders for generating fluid pressure in vehicle hydraulic brake systems have generally included a reservoir, a cylinder, an intake port and a by-pass port providing fluid communication between the reservoir and the cylinder, a piston movably positioned in the cylinder, a primary sealing cup also movably positioned in the cylinder adjacent the forward end of the piston, and a secondary sealing cup carried by the rearward end of the piston. The primary and secondary sealing cups are generally formed of resilient material and have their annular peripheries disposed in sliding and sealing engagement with the annular wall of the cylinder. When the piston is fully retracted in the cylinder, the primary sealing cup is located ahead of the intake port and behind the by-pass port so that fluid seepage and fluid temperature expansion or contraction in the brake system may be compensated for by fluid flow between the cylinder and the reservoir via the bypass port.
When the vehicle brake pedal is depressed, the piston is moved forward in the cylinder thereby moving the primary sealing cup forward past the bypass port. The piston then generates fluid pressure in the brake system for actuating the vehicle wheel brakes. If the fluid pressure increase during the initial forward movement of the piston is significant, such as in disc brake systems, a portion of the annular periphery of the primary sealing cup will probably be extruded into the by-pass port as the primary sealing cup passes over the by-pass port. The edge of the by-pass port then very often cuts off the extruded portion of the primary sealing cup. This is commonly known as cup cutting and results in fluid leakage past the annular periphery of the primary sealing cup which may cause brake failure due to the inability of the master cylinder to generate sufficient fluid pressure to actuate the brakes.
U.S. Pat. No. 3,955,371 shows two master cylinder designs which eliminate this problem of cup cutting. Both master cylinder designs include a secondary piston movably positioned in a secondary cylinder which is formed within the main piston. The secondary cylinder is in open fluid communication with the main cylinder. In one design, the secondary piston is moved rearward in the secondary cylinder as the main piston is moved forward in the main cylinder. A drawback of this design is that the rearward movement of the secondary piston causes fluid pressure surges in the main cylinder between the primary and secondary sealing cups which could result in fluid leakage past the secondary sealing cup. In the other design, the secondary piston is initially moved forward in the secondary cylinder while the main piston remains stationary in the main cylinder. This design has a drawback in that the initial forward movement of only the secondary piston also causes fluid pressure surges in the main cylinder between the primary and secondary sealing cups which could result in failure of the secondary sealing cup.
It is, therefore, an object of this invention to provide an improved master cylinder which is noncup cutting yet overcomes the aforementioned drawbacks of the previous non-cup cutting master cylinder designs.